Grain-oriented silicon steel sheets are used as magnetic cores of various electric appliances. Grain-oriented silicon steel sheets are steel sheets, containing Si at 0.8% to 4.8%, which have crystal grains highly oriented in the {110}<001> direction. The required magnetic properties are high magnetic flux densities (represented by the value of B8) and low iron losses (represented by the value of W17/50). Recently, due to an increasing concern for energy conservation, particularly, the demand for lower power losses is increasing.
To meet these requirements, technologies to finely divide magnetic domains have been developed as means for decreasing iron losses of grain-oriented silicon steel sheets.
In a case of producing stacked iron cores, Japanese Examined Patent Publication (Koukoku) No. 58-26405, for example, discloses a method for decreasing iron losses by finely dividing magnetic domains by localized strains which are introduced by irradiating laser beams onto finish-annealed sheets.
Observation of the movement of the finely divided magnetic domains, however, revealed that some of magnetic domains are pinned and made stationary by the asperity of the glass coating on the surface of steel sheets. In order to further decrease iron losses of grain-oriented electrical steel sheets, therefore, it is considered important to diminish the pinning effect caused by the asperity of the glass coating on the surface of steel sheets that hampers the motion thereof, in addition to fine dividing of magnetic domains.
Not forming a glass coating that hampers the motion of magnetic domains, on the surface of steel sheets, is considered effective. The specification of the U.S. Pat. No. 3,785,882, for example, discloses a method not forming glass coating and using coarse high-purity alumina as an annealing separator. As, however, this method cannot eliminate inclusions existing immediately below the surface, the improvement in iron loss remains not more than 2% in terms of W15/60 because of the pinning effect of such inclusions.
Japanese Unexamined Patent Publication (Kokai) No. 64-83620, for example, discloses a method of applying chemical or electrolytic polishing, after finish-annealing, as means for holding back the production of inclusions immediately below the surface and providing smooth (mirror-like) surfaces. Chemical and electrolytic polishing, however, have been possible only in processing small specimens on a laboratory scale. They have not been used practically because there are difficult problems in control of chemicals' concentration and temperature and in the provision of pollution control equipment.
To solve the above problems, the inventors made various experiments and found that control of the dew point of decarburized-annealing and prevention of the formation of Fe-based oxides (such as Fe2SiO4 and FeO) in the oxidized layer formed in the course of decarburized-annealing are effective for elimination of surface inclusions (refer to Japanese Unexamined Patent Publication (Kokai) No. 7-118749).
Application of an aqueous slurry, or dry coating by electrostatic or other methods, of an annealing separator consisting mainly of alumina on decarburized-annealed sheets having an oxidized layer provides a mirror-like surface after finish-annealing and thereby greatly decreases iron losses.